A multijoint arm type robot is provided with an arm which includes a plurality of arm units and a plurality of joints for rotatably connecting the distal end of one arm unit to the proximal end of another arm unit. Each arm unit is provided with actuators at the distal end and the proximal end. Each actuator has a motor and a reduction gear device. When the motor is driven, the arm unit rotates around the joint acting as a center of rotation. Since the arm unit requires a high torque in order for it to rotate, the reduction gears are therefore required to have a high reduction ratio. Since the weight of arm units situated at the distal end of an arm exerts a considerable load on the actuators of arm units situated at the proximal end, therefore, the robot should be built small in size and light in weight, as should also the reduction gear device. When the arm is stationary, it must be accurately positioned so as not to move even slightly. Accordingly, the reduction gear device should have high torsional rigidity and high power transmission characteristics.
As the above-described reduction gear device, a differential planetary gear apparatus shown in FIG. 1 has therefore been employed. The apparatus has an input shaft 1 and an output shaft 6. A sun gear 2 is fixed to the input shaft 1. A plurality of planet gears 4 are disposed at equal interval circumferentially around the sun gear 2. The planet gears 4 engage with a fixed internal gear 3 and a rotary internal gear 5. The rotary internal gear 5 is coupled with the output shaft 6.
When the input shaft 1 is rotated, the sun gear 2 is rotated and each of the planet gears 4 revolves around the sun gear 2 while revolving around its own axis. Thus, the rotary internal gear 5 is rotated by the angle determined by the difference in the number of the teeth of the rotary and fixed internal gears 5 and 3. As a result, the output shaft 6 is rotated at a reduced rotational frequency with respect to that of the input shaft 1.
As has already been stated above, the planet gears 4 are disposed at equal intervals in a revolving orbit around the sun gear 2. Therefore, the loads exerted by the internal gears 3 and 5 are uniformly distributed among the planet gears 4, to improve the power transmission characteristics and torsional rigidity. Each planet gear 4 comprises a portion C engaged with the fixed internal gear 3 and a portion D engaged with the rotary internal gear 5, as is shown in FIGS. 2A and 2B. Portions C and D have no phase difference. These portions are formed integrally. Further, either or both of the internal gears 3 and 5 is profile-shifted so that the diameters of the addendum circles of the gears 3 and 5 are the same size.
The reduction gear ratio R of the differential planetary gear apparatus is obtained by the following equation, in the case of Z.sub.C &lt;X.sub.D where Z.sub.A, Z.sub.C, and Z.sub.D represent, respectively, the number of the teeth of the sun gear 2, of the fixed internal gear 3, and of the rotary internal gear 5. EQU R={1+(Z.sub.C /Z.sub.A)}/{1-(Z.sub.C /Z.sub.D)} (1)
As is apparent from equation (1), a large reduction gear ratio can theoretically be freely set. For example, when the difference M=.vertline.Z.sub.D -Z.sub.C .vertline. in number of the teeth between both internal gears is reduced, the reduction gear ratio increases. When the difference M of the numbers of the teeth is increased, the reduction gear ratio decreases.
However, in order to dispose planet gears at an equal interval circumferentially around the sun gear, it is necessary to satisfy "the assembling conditions", in which the phases of teeth of the respective planet gears coincide with those of spaces of the gears engaged with the planet gears. More specifically, N planet gears are disposed at equal intervals circumferentially by being divided equally at 2.pi./N. To this end, it is necessary that the number of the teeth of the gears engaged with the planet gears should be exactly divisible by N. Therefore, when the numbers Z.sub.A, Z.sub.C, Z.sub.D of the teeth of the respective gears are selected, Z.sub.A, Z.sub.C, and Z.sub.D must be set to the integral multiple of number N of the planet gears arranged at equal intervals. For example, when the difference M in the numbers of teeth is M=1, the number N of the planet gears becomes N=1. However, when N=3 is selected in the case of M=1, the phase of the teeth of one planet gear coincides with the phase of the spaces of the internal gears, but the phases of the teeth of the remaining two planet gears do not coincide with that of the spaces of the internal gears. Thus, since the number N is not freely set, a desired reduction gear ratio may not always be obtainable, as will be described.
For example, when the value of the difference M=.vertline.Z.sub.D -Z.sub.C .vertline. of the teeth of the gears is 1 or 2, the number N of the planet gears is small, i.e., 1 or 2. Thus, the loads exerted by the fixed and rotary internal gears are unevenly distributed among the small number of planet gears, thereby causing a reduction in the torsional rigidity. Thus, the case wherein N=1 cannot always be realized. On the other hand, when difference M of the teeth of the gears is 4 or 5, number N is relatively great, i.e., 4 or 5. Thus, there arises the drawback wherein the planet gears cannot be accommodated on the revolving orbit around the sun gear or they interfere with each other.
Therefore, the number N of the planet gears N=3 is most preferable. In this case, the difference M in number of the teeth between the gears cannot be other than a multiple of 3, according to the assembling conditions. As has been explained above, since the conventional differential planetary gear apparatus is restricted as to the selection of the number (i.e., the equal interval distribution number N) of the planet gears, the selections of the numbers of the teeth of the gears are limited, with the result that a variety of reduction gear ratios cannot be provided.
It is, therefore, desired that N be 3, and a variety of reduction ratios (or speed ratios) be provided. The condition desired in practice is that M=1 and N=3. This condition cannot be satisfied by the conventional differential planetary gear apparatus.